In general, an electrocardiographic (electrocardiogram) signal obtained by differentially amplifying electrical signals, which are generated with motions of the cardiac muscle, and a photo-plethysmographic signal obtained by optically detecting the blood pulsation in the artery, which is caused with beat pulses, are used for diagnosis of cardiovascular diseases, etc.
Recently, a biosensor device has been developed which has the function of detecting the electrocardiographic signal and the photo-plethysmographic signal at the same time and obtaining biological information, e.g., a heart rate, an oxygen saturation, and a pulse-wave propagation time. In such a biosensor device, an optical probe for detecting the photo-plethysmographic signal is disposed on one or both of two electrodes for detecting electrical signals relating to the electrocardiographic signal. The optical probe includes a light emitter and a light receiver, which are mounted in recesses or holes formed in the surface of the electrode.
In that type of related-art biosensor device, when a user puts fingers on the surfaces of the electrodes, electrical signals are detected from the fingers through the electrodes, and an electrocardiographic signal is obtained from the detected electrical signals. Simultaneously, detection light is emitted to the finger from the light emitter mounted in the recess or the hole formed in the electrode surface, and reflected light after the detection light has been reflected at the finger is received by the light receiver mounted in the recess or the hole formed in the electrode surface. A photo-plethysmographic signal corresponding to the received reflected light is then obtained (see Patent Document 1).
On the other hand, for the purpose of increasing an SN (Signal to Noise) ratio of the photo-plethysmographic signal obtained from the user's finger, it is desirable to restrict divergence of the detection light emitted from the light emitter, and to effectively collect the detection light toward the user's finger that is put on the electrode.
As a manner for collecting light emitted from a light emitting diode in a particular direction, there is known a technique of providing, on a substrate, a reflector having a concave reflecting surface, and mounting the light emitting diode at the bottom of a concave recess of the reflector (see Patent Document 2). Further, there is known a technique of providing a metal ring on the substrate in surrounding relation to the light emitting diode, forming a fillet portion made of an Ag (silver)-based brazing alloy along an inner peripheral surface of the ring, and utilizing the fillet portion as a light reflecting surface (see Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-158974    Patent Document 2: Japanese Unexamined Utility Model Application Publication No. 62-79291    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2005-244121
Meanwhile, one conceivable approach for promoting a size reduction of a biosensor device, which can detect the electrocardiographic signal and the photo-plethysmographic signal at the same time, is to reduce the size of each of the electrodes for detecting the electrical signals relating to the electrocardiographic signal. However, when the electrode size is reduced, contact between a living body (such as a user's finger) and the electrode become unstable and the SN ratio of the electrocardiographic signal is reduced.
Another conceivable approach for promoting the size reduction of the biosensor device is to shorten the distance between the light emitter and the light receiver for detecting the photo-plethysmographic signal. However, when the distance between the light emitter and the light receiver is shortened, the SN ratio of the photo-plethysmographic signal is reduced.
In more detail, the reflected light received by the light receiver contains reflected light having passed through the artery under the skin of the living body, and reflected light having been reflected at an outer surface of the skin of the living body. The reflected light having passed through the artery under the skin of the living body contains a pulse component of the artery blood, and an AC signal component corresponding to the pulse component is used as the photo-plethysmographic signal. On the other hand, the reflected light having been reflected at the outer surface of the skin of the living body does not contain the pulse component of the artery blood, and a signal component corresponding to the relevant reflected light is almost a DC component.
When the distance between the light emitter and the light receiver is shortened, a ratio of the reflected light having passed through the artery under the skin of the living body to the reflected light having been reflected at the outer surface of the skin of the living body is relatively reduced in the reflected light received by the light receiver. As a result, a ratio of the AC signal component useful as the photo-plethysmographic signal to the DC signal component not useful as the photo-plethysmographic signal is relatively reduced in an electrical signal that is obtained by converting the reflected light received by the light receiver. Hence, an influence of noise upon the photo-plethysmographic signal is increased and the SN ratio of the photo-plethysmographic signal is reduced.
In the related-art biosensor device disclosed in the above-cited Patent Document 1, because the light emitter and the light receiver are mounted in the recesses or the holes formed in the electrode surface, the electrode surface includes a concave-convex configuration and the concave-convex configuration makes unstable the contact between the user's finger, for example, and the electrode. This raises a risk that the SN ratio of the electrocardiographic signal may be reduced. Further, in the related-art biosensor device, because a light emitting portion of the light emitter and a light receiving portion of the light receiver are exposed at the electrode surface, it is difficult to protect those exposed portions against externally applied friction and shocks.
Additionally, as described above, for the purpose of increasing the SN ratio of the photo-plethysmographic signal obtained from, e.g., the user's finger, it is desirable to restrict divergence of the detection light emitted from the light emitter, and to effectively collect the detection light toward the user's finger that is put on the electrode.
However, when trying to realize the above point by providing the reflector or the ring on the substrate as disclosed in the above-mentioned related art, an area of the substrate is increased and a difficulty arises in reducing the size of the biosensor device. Further, because a step of providing the reflector or the ring on the substrate needs to be added to a manufacturing process for the biosensor device, and the manufacturing cost of the biosensor device is increased.
When the technique of forming a fillet portion made of an Ag (silver)-based brazing alloy along an inner peripheral surface of the ring is employed as disclosed in the above-cited Patent Document 3, heat treatment at high temperature of 600° C. or higher is required to form the fillet portion. Hence, the following problems occur. The disclosed technique cannot be applied to a printed circuit board, for example. The blazing has to be carried out before mounting elements, which constitute an electrical circuit, on the substrate. Equipment for high-temperature heat treatment is required.